The present invention generally relates to a method of controlling the operation of an automatic, non-invasive blood pressure (NIBP) monitor. More specifically, the present invention relates to a method of controlling the operation of a NIBP monitor to utilize both a linear deflation technique and step deflation technique to determine blood pressure.
Automated blood pressure monitors employ an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith and incorporated by reference, employs the oscillometric methodology.
In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure above the systolic pressure. The cuff pressure is then reduced in predetermined decrements, and at each level, pressure fluctuations are monitored. The resultant arterial pulse signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as “oscillation complexes” or just simply “oscillations”). The oscillation amplitudes measured from the cuff can range from a fraction of a mmHg to as much as 8 mmHg.
After suitable filtering to reject the DC component and to provide amplification by a scale factor, peak oscillation amplitudes measured above a given base-line are stored. As the cuff pressure decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. These amplitudes form an oscillometric envelope for the patient. The lowest cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure (MAP) of the patient. Systolic and diastolic pressures can be derived either as predetermined fractions of the oscillation size at MAP, or by more sophisticated methods of direct processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. When in use, the blood pressure cuff is placed on the patient and the operator usually sets a time interval, typically from 1 to 90 minutes, at which blood pressure measurements are to be made. The noninvasive blood pressure (NIBP) monitor automatically starts a blood pressure determination at the end of the set time interval.
Generally, conventional NIBP monitors of the type described in the afore-mentioned patents use oscillation pulse amplitude matching at each pressure level as one of the ways to discriminate good oscillations from artifacts. In particular, pairs of oscillation pulses are compared at each pressure level to determine if they are similar in amplitude and similar in other attributes, such as shape, area under the oscillation curve, slope, and the like. If the oscillation pulses compare within predetermined limits, the average pulse amplitude and cuff pressure are stored and the pressure cuff is deflated to the next pressure level for another oscillation measurement. However, if the oscillation pulses do not compare favorably, the attributes of the earlier oscillation are typically ignored and the attributes of the latter oscillation are stored. The monitor does not deflate; instead, the monitor waits for another oscillation to compare with the one that was stored. This process continues until two successive oscillation pulses match or a time limit is exceeded.
Although the step deflation technique described above can eliminate or reduce the effect artifacts have in the blood pressure determination, the step deflation technique typically requires the detection of two oscillation pulses during each pressure step. Sometimes under artifact free circumstances an attempt can be made to obtain only one pulse at each step; however, there are still time inefficiencies even in this case. Even when the detected oscillation pulses are very clean and artifact free, the step deflation technique has an inherent delay in order to control the pressure level of each step. Therefore, the amount of time required to make a blood pressure determination will be extended by the time that the technique uses at each pressure step to control the pressure.
An alternate method of obtaining a blood pressure measurement is to operate the NIBP monitor using a continuous deflation from an initial inflation pressure to a final pressure. Typically, the recommended continuous deflation pattern is linear. During the linear deflation, the cuff pressure is decreased at a specific rate (mmHg/second) and the oscillation pulse amplitudes are measured for the cuff pressure as the pressure is continuously decreased. Since, in the case when the oscillometric signal is not corrupted by artifact, the NIBP system does not need to maintain pressure at a defined step to obtain high quality pulses, an NIBP system utilizing the linear deflation technique can often obtain a blood pressure measurement more quickly than a system utilizing the step deflation technique. However, note that other factors, like pulse pressure and heart rate, do influence the time it takes to complete a blood pressure determination for either the linear or step deflate patterns.
However, since the pressure of the blood pressure cuff is deflated continuously, if any one of the oscillation pulses is inaccurate due to an artifact introduced by the patient or some other external variable, the linear deflation technique does not include a mechanism to compare recorded oscillation pulse amplitudes, as is possible when utilizing the step deflation technique. Therefore, it can be understood that operating an NIBP monitor utilizing either a step deflation technique or a linear deflation technique has relative drawbacks in certain types of situations. Selectively operating an NIBP monitor utilizing both the step deflation technique and the linear deflation technique would be an improvement over the state of the art.